So today, we're going
to talk about enzymes and all the different
kinds of reactions that enzymes can catalyze. But before we do that,
let's review the idea that enzymes make biochemical
reactions go faster. And if you look at a
reaction coordinate diagram, you'd notice that enzymes
speed up reactions by lowering their
activation energy. Now, enzymes are generally
named for their reactions, which is convenient because it makes
it a lot easier to remember what an enzyme does if
someone gives you its name. And a great example
of this is that one of the enzymes involved
in DNA replication is called DNA polymerase, which
is named as such because it acts on DNA and specifically
makes polymers of DNA. Now, the suffix "ase"
is usually just one that you find at the end
of most enzyme names. Now, another great example
is that the enzyme that catalyzes the first step of
glycolysis, which you may remember is the reaction
between glucose and ATP to form glucose-6-phosphate
and ADP, is called hexokinase. And "hexo" refers
to the number 6, which is a reference to glucose
being a six-carbon sugar. And "kinase" is a term
referring to enzymes that add phosphate functional
groups to different substrates. So overall, hexokinase
adds phosphates to six-carbon
sugars like glucose. Now generally, every enzyme
has a very specific name that gives insight into
the specific reaction that that enzyme can catalyze. So we can actually
divide most enzymes into six different
categories based off the kinds of reactions
that they catalyze. Now, our first group is
the transferase group. And the basic reaction
that transferases catalyze are ones where you move
some functional group, X, from molecule B to molecule
A. And a great example of one of these reactions occurs
during protein translation, where amino acids
bound to tRNA molecules are transferred over to the
growing polypeptide chain. So in this case, A refers
to our amino acid chain, B refers to our tRNA, and X
refers to this lysine residue, which is being
transferred from B to A. And this reaction
in particular is catalyzed by an enzyme called
peptidyl transferase, which is an appropriate name since
it is a transferase involved in making peptides. Next we have the
ligase group, which catalyzes reactions
between two molecules, A and B, that are combining to
form a complex between the two, or AB. And an example of a
reaction using a ligase that you might be familiar with
occurs during DNA replication, where two strands of DNA
are being joined together. So in this reaction, A and B
represent the two separated DNA polymers, which are being
joined to form a single strand. And this reaction
in particular is catalyzed by an
enzyme called DNA ligase, which is
named since it's a ligase that works
on DNA strands. Now our third group as
the oxidoreductase group, which is a little
different from the others since it actually includes two
different types of reactions. And these reactions involve
transferring electrons from either molecule
B to molecule A or from molecule A
to molecule B. Now, we say that an oxidase
is directly involved in oxidizing or taking
electrons away from a molecule, while a reductase is involved
in reducing or giving electrons to a molecule. And we call these enzymes
oxidoreductases together because they can
usually catalyze both the forward and
reverse reactions, which is why I used equilibrium
arrows here instead of just a normal single-headed arrow. Now a great example of an
oxidation reduction reaction occurs during lactic acid
fermentation, where electrons are either passed from NADH to
pyruvate or from lactic acid to NAD. Now, this reaction is
catalyzed by an enzyme called lactate dehydrogenase. Remember that the
word "dehydrogenase" refers to the removal of a
hydride functional group. And that's the same as saying
the removal of electrons, since hydrides are
basically just hydrogen atoms with two electrons on
them instead of just one. Now, this enzyme
is given its name since it's able to remove a
hydride, or remove electrons, from a molecule of lactic acid. Next, we have the
isomerase group. And enzymes in this
group are typically involved in reactions where
a molecule, like molecule A, is being converted to
one of its isomers. And an example of this
type of a reaction is the conversion of
glucose-6-phostate to fructose-6-phosphate,
which is one of the steps of glycolysis
that you may remember. Now, this reaction is
catalyzed by an enzyme called phosphoglucose isomerase,
which is appropriately named since it creates
isomers of glucose molecules that are phosphorylated. Now, our next category is
the hydrolase category. And hydrolases use
water to cleave a molecule, like molecule A,
into two other molecules, B and C. And a great example
of one of these reactions is the hydrolysis reaction that
can occur to peptide bonds. And if we have this
lysine-alanine dipeptide here, it could be reacted
with water to form two individual amino acids
that are no longer bound. And this particular
hydrolysis reaction can be catalyzed by
a class of enzymes that we call serine
hydrolases, which some people call
serine proteases. And they are named this way
because they are hydrolases that use a serine residue as the
key catalytic amino acid that is responsible for
breaking the peptide bond. Now, our last category is
a little more complicated than the others. And it's the lyase group. Now, lyases catalyze the
dissociation of a molecule, like molecule A, into
molecule B and C, without using water
like hydrolases would, and without using
oxidation or reduction like an oxidoreductase would. And one example of a
reaction catalyzed by a lyase is the cleavage of
argininosuccinate into arginine and succinate. And this reaction takes
place during the urea cycle, which you also might
be familiar with. Now this specific reaction is
catalyzed by an enzyme called argininosuccinate lyase,
which is appropriately named because it is a
lyase that catalyzes the breakdown of an
argininosuccinate molecule. Now, it's important to recognize
that since lyases don't use water or oxidation
to break a bond, they need to generate either a
double bond between two atoms or a ring structure in a
molecule in order to work. So what did we learn? Well, first we learned
that enzymes are sometimes named for their reactions. And next we learned about the
six different types of enzymes. We have transferases, which
transfer functional groups from one molecule to another;
ligases, which ligate or join two molecules together;
oxidoreductases, which move electrons
between molecules; isomerases, which
convert a molecule from one isomer to
another; hydrolases, which break bonds using water;
and lyases, which break bonds without using water and
without using oxidation.